The number of HCV-infected people in the world is estimated to be one to two hundred million, with two million or more estimated to be infected in Japan. Approximately 50% of these patients undergo a transition to chronic hepatitis; thirty or more years after infection, approximately 20% of these will develop hepatic cirrhosis and liver cancer. Approximately 90% of liver cancers are said to be caused by hepatitis C. In Japan, 20,000 or more patients die every year due to liver cancers associated with HCV infection.
HCV was discovered in 1989 as a major causative virus of post-transfusion non-A and non-B hepatitis. HCV is an enveloped RNA virus whose genome consists of a single-stranded (+) RNA, and is classified into the genus Hepacivirus of the family Flaviviridae.
HCV evades the host immune mechanism for an as yet unknown reason. Therefore, even when adults with a mature immune mechanism are infected, persistent infections are established in many cases, and this develops into chronic hepatitis, liver cirrhosis, and liver cancer. Even when the cancers are removed by surgery, many patients are known to have recurring liver cancer due to the inflammation continuously induced in non-cancerous parts.
The establishment of effective therapeutic methods for hepatitis C is therefore desired. In addition to response therapies that suppress inflammation using anti-inflammatory agents, the development of pharmaceutical agents that reduce or eradicate HCV in the liver, which is the affected area, is particularly strongly desired.
Currently, interferon treatment is known to be the only effective therapeutic method for eliminating HCV. However, interferons are effective for approximately one third of all patients. The effectiveness of interferons against HCV genotype 1b is particularly low. There is therefore a strong desire to develop anti-HCV agents that may be used instead of, or together with, interferons.
In recent years, Ribavirin (1-β-D ribofuranosyl-1H-1,2,4-triazole-3-carboxamide) has been commercially available as a therapeutic agent for hepatitis C for combined use with interferons; however, its efficacy ratio is still low and additional novel therapeutic agents for hepatitis C are desired. Attempts have also been made to eliminate the virus by strengthening patient immune systems using interferon agonists, interleukin-12 agonists, and such, but effective pharmaceutical agents have not yet been found.
Since cloning the HCV gene, molecular biological analyses of the mechanism and function of the viral gene, the function of each of the viral proteins, and such has progressed rapidly; however, the mechanisms of replication, persistent infection, pathogenicity, and such of the virus in host cells have not been sufficiently clarified, and experimental HCV infection systems using reliable cultured cells have not been constructed. Therefore, evaluation of anti-HCV agents has conventionally required alternative viral assay methods that use other related viruses.
In recent years, however, it has become possible to observe HCV replication in vitro using non-structural regions of HCV, and replicon assay methods have enabled easy evaluation of anti-HCV agents (Non-Patent Document 1). The mechanism of HCV RNA replication in this system is considered identical to the replication of a full length HCV RNA genome that has infected liver cells. This system can therefore be a cell-based assay system useful for identifying compounds that inhibit HCV replication.
Prior art references relating to the present invention of this application are shown below.    [Patent Document 1] International Publication WO98/56755 pamphlet    [Patent Document 2] Japanese Patent Application No. 2003-34056    [Patent Document 3] Japanese Patent Application No. 2003-272420    [Non-Patent Document 1] V. Lohmann et al., Science (1999) 285:110-113.